In general, a filter within an electrical circuit allows selected signals to xe2x80x9cpassxe2x80x9d while blocking other signals. One type of filter is a bandpass filter. A typical bandpass filter is an electrical device or circuit that allows signals in a specific frequency range to pass, but that blocks signals at other frequencies.
Bandpass filters are frequently used in electrical circuits in devices such as radio, television, cordless and cellular telephones, wireless communications systems, radar, sensors, and some types of manufacturing measurement and instrumentation systems. These devices transmit and receive signals using electromagnetic waves.
A primary function of a bandpass filter in a transmitter is to limit the bandwidth of the output spectrum. In a receiver, a bandpass filter allows the receiver to receive a selected range of frequencies, while rejecting signals at unwanted frequencies. A bandpass filter also optimizes the signal-to-noise (sensitivity) of a receiver. In both transmitting and receiving applications, well-designed bandpass filters, having the optimum bandwidth for the mode and speed of communication being used, maximize the number of signals that can be transferred in a system, while minimizing the interference or competition among signals.
An example of an application of filters in electronics is in microwave communications, that is, wireless communications using signals in the microwave portion of the electromagnetic spectrum. Conventional filter designs intended to operate at high frequencies include edge-coupled, surface acoustic wave (SAW), dielectric resonator and waveguide filters. Another type of conventional filter used in microwave communications is a filter having two square loop resonators where the square loop resonators are positioned on either side of a core material where the loops are off-center from each other. This type of filter can be realized in two layers of a printed wiring board, for example. In operation, the square loop resonators cross-couple with each other thereby each influencing the electrical response of the other to produce a signal useful in microwave communications. The response of this filter is controlled by varying the amount of offset in the relative positions of the resonators.
Conventional filter design and operation suffers from a variety of difficulties. For example, conventional signal filtering technology typically does not filter well where unwanted frequencies are close to a selected pass frequency. This often causes difficulty in blocking the unwanted signal. Filters in these situations are typically used to band-limit thermal noise and to reject image frequencies and other close-in spurious signals. The requirements for high frequency bandpass filters typically include a compact topology, a narrow, sharp passband, high rejection at close-in frequencies and overall inexpensive fabrication and tuning. In the above-described conventional edge-coupled, surface acoustic wave (SAW), dielectric resonator and waveguide filters, the resonator topologies have relatively high fabrication costs and are bulky and difficult to tune. It remains desirable to have a method and apparatus for a bandpass filter having high selectivity for passing a desired frequency while filtering close-in undesirable frequencies.
Embodiments of the present invention significantly overcome such deficiencies by providing techniques for filtering which use a novel filtering structure having a coupled ring resonator topology. Such a structure is well-suited for bandpass filtering because it provides a high close-in rejection elliptic-response. Such a structure yields filters that are small and narrow-band. Further, this topology is advantageous in that it can be realized using relatively inexpensive standard lithography techniques.
More specifically, embodiments of the invention provide methods and apparatus that use ring resonator pairs placed orthogonally to feed lines in a filter circuit. The feed lines are split in order to couple with two resonator pairs. The resonators in each resonator pair couple with each other as well as with the feed lines. Resonator placement in relation to other resonators and resonator coupling length are used to tune the filter circuit in order to pass selected frequencies. Resonator placement in relation to feed lines and width of the resonator are also used to tune the filter circuit. In one embodiment, this topology effectively forms an Elliptic Function response bandpass filter with high close-in rejection capability. Further, these topologies of embodiments of the invention may be relatively inexpensively produced standard lithography techniques such as those used in printed wiring board manufacturing or in thin film manufacturing.
One such embodiment of a filter includes a first feed line having a first stem connected to a first coupling extension and a second coupling extension and a second feed line having second stem connected to a third coupling extension and a fourth coupling extension where the first coupling extension is substantially parallel to the third coupling extension and the second coupling extension is substantially parallel to the fourth coupling extension. The embodiment further includes four ring resonators located planar to the first and the second feed lines, two of the ring resonators being positioned between the first and the third coupling extensions and two other of the ring resonators being positioned between the second and the fourth coupling extensions such that the four ring resonators are coupled to the feed lines to form a first resonant circuit. The resonant circuit of this topology provides a well-defined passband where close-in frequencies can be blocked while passing a signal of a selected frequency.
In another embodiment of the invention, each ring resonator is substantially one-quarter wavelength (xcex/4) on each side providing a passband that is substantially centered on the selected frequency. Accordingly, the filter can be centered about a particular frequency by scaling the resonator lengths proportionally to wavelength.
In another embodiment of the invention, each ring resonator is a square-shaped ring. Square-shaped rings couple more effectively with the feed-lines and with each other than rounded ring resonators. In another embodiment of the invention, each side of the square-shaped rings is substantially one quarter wavelength (xcex/4) of the selected center frequency. Further, the portion of each the coupling extension that is adjacent to a side of each square-shaped resonator forms a coupling point, and each coupling point is a quarter wave coupler. This provides balanced resonance throughout the resonant circuit.
In another embodiment of the invention, the feed-lines, coupling extensions and ring resonators are conductive features of a signal layer on a substrate. In these embodiments of the invention, the bandpass filter formed using the disclosed inventive features is part of a circuit.
In another embodiment of the invention, the bandpass filter further includes a fifth ring resonator between the first and the third coupling extensions and a sixth ring resonator between the second and the fourth coupling extensions. Additional resonators in the bandpass filter improve the definition of the passband.
In another embodiment of the invention, the ring resonators are positioned relative to each other such that each the ring resonator affects resonance in adjacent resonators such that the passband of the bandpass filter is defined. The positioning the ring resonators relative to each other effectively tunes the bandpass filter.
In another embodiment of the invention, bandpass filter is configured to operate in the radio frequency region of the electromagnetic spectrum. In this way, the features of the bandpass filter are re-sized according to a selected frequency from the radio frequency range.
In another embodiment of the invention, a first resonant circuit having ring resonators and a second resonant circuit having ring resonators as described above are connected in series. This is also referred to as xe2x80x9ccascadingxe2x80x9d the filters. The filters connected in series provide an even sharper passband than one filter alone.
Method embodiments of the invention include a method of filtering, including the steps of providing a first feed line and a second feed line, providing a pair of ring resonators positioned orthogonal to the feed lines, and providing coupling extensions on each feed line such that the feed lines couple to the a pair of ring resonators to form a first resonant circuit.
In another embodiment of the invention, the method further includes placing the at least two ring resonators in relation to each other such that each the ring resonator affects resonance in adjacent resonators such that a passband of the bandpass filter is defined whereby the bandpass filter is tuned.
In another embodiment of the invention, the method further includes the step of configuring the first and the second feed-lines, the at least two pairs ring resonators and the coupling extensions to operate at radio frequencies.
In another embodiment of the invention, the method further includes forming the first and second feed lines, the at least two ring resonators and the coupling extensions as features on a plane of a substrate using printed wiring board technology. In another embodiment of the invention, the method further includes forming the first and second feed lines, ring resonators and the coupling extensions as features on a plane of a substrate using thin film techniques. In this way, the bandpass filters can be constructed using relatively inexpensive, standard manufacturing techniques thus making them relatively inexpensive and easy to implement.